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Virginia Institute of Marine Science

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Adult infaunal clams (Macoma balthica) persist at low densities in sandy and muddy habitats in Chesapeake Bay, USA, despite intense predation by blue crabs Callinectes sapidus; another infaunal soft-shelled clam (Mya arenaria) only persists in sandy habitats. We hypothesized that the persistence of M. balthica and M. arenaria in certain habitats was due to blue crabs exhibiting a type III (sigmoid) functional response whereby the risk of mortality is reduced at low clam densities. Laboratory experiments assessed functional responses (prey consumption predator-1 as a function of prey density) of large male blue crabs to 6 densities of M. balthica as a function of sediment type (sand and mud) and tank size (54.6 cm and 97.2 cm diameter). These results were compared with previous experiments with M. arenaria. Abundances of blue crabs, M. balthica, and M. arenaria were measured 4 to 10 times yr-1 from 1979 to 1986 at both sand and mud stations in the mesohaline zone of Chesapeake Bay. Laboratory functional response results were then related to seasonal habitat-specific abundance patterns of M. balthica and M. arenaria in the field. With the exception of M. balthica in mud, abundances of M. arenaria and M. balthica decreased as blue crab abundance increased during the summer. When blue crabs reached their peak abundances in July, M. balthica was predominant in mud whereas M. arenaria numbers dropped to zero in mud and persisted in sand at similar low densities to M. balthica in sand. In the laboratory, blue crabs exhibited density-dependent (type III) functional responses to M. balthica irrespective of sediment type and tank size, even though mortality rates of clams were significantly higher in sand than mud. Differences in habitat-specific burial depth probably accounted for the differential survival of M. balthica in sand and mud. Thus, M. balthica obtained a relative refuge from blue crab predation at low densities similar to those in the field near the end of the seasonal period of active predation. Previous laboratory experiments with blue crabs indicated a type III functional response to M. arenaria in sand and an inversely density-dependent type II response to M. arenaria in mud. Thus, the collective laboratory and field evidence from this study and others strongly suggests that blue crabs are critical determinants of species- and habitat-specific prey persistence in marine soft-bottom communities, and that analysis of predator functional and aggregative responses may help to explain much of the spatial variation of clam abundance patterns in the mesohaline zone of Chesapeake Bay.